Selective amplification of overlapping amplicons

ABSTRACT

The present invention relates to a scalable multiplex PCR method that can simultaneously amplify overlapping amplicons without the drawbacks of conventional multiplex PCR. The method selectively amplifying target nucleic acid fragments having an overlapping region. The method comprises the steps of: obtaining a first nucleic acid sequence comprising a first tag t2 and a first forward primer F1, obtaining a second nucleic acid sequence comprising a second tag t1 and a first reverse primer R1, obtaining a third nucleic acid sequence comprising the second tag t1 and a second forward primer F2, obtaining a fourth nucleic acid sequence comprising a third tag t3 and a second reverse primer R2, wherein each primer is a gene-specific primer; performing initial cycles of PCR; and then performing later cycles of PCR at higher annealing temperatures to obtain amplification products.

This application is continuation-in-part of U.S. application Ser. No. 15/057,343, filed Mar. 1, 2016; which claims priority to U.S. Provisional Application No. 62/129,360, filed Mar. 6, 2015; the content of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

This invention relates to a method for selectively amplifying target nucleic acid fragments having an overlapping region. The present method enriches copies of target to amplicons over copies of the overlapping regions of amplicons.

REFERENCE TO SEQUENCE LISTING, TABLE OR COMPUTER PROGRAM

The Sequence Listing is concurrently submitted herewith with the specification as an ASCII formatted text file via EFS-Web with a file name of Sequence Listing.txt with a creation date of Dec. 21, 2016, and a size of 5.53 kilobytes. The Sequence Listing filed via EFS-Web is part of the specification and is hereby incorporated in its entirety by reference herein.

BACKGROUND OF THE INVENTION

Multiplex-PCR consists of multiple primer sets within a single PCR mixture to produce amplicons that are specific to different DNA sequences. By targeting multiple genes at once, additional information may be gained from a single test run that would otherwise require several times of the reagents and more time to perform. Annealing temperature for each of the primer sets must be optimized to work correctly within a single reaction. Commercial kits for multiplexing PCR general reagents are available. The technique of multiplex PCR has been used for target enrichment for next-generation sequencing (NGS), which refers to high throughput parallel DNA sequencing technologies. Millions or billions of DNA strands can be sequenced concurrently, yielding substantially high throughput.

One major reason for amplicon drop-out is preferential amplification of the short overlapping regions between two overlapping amplicons during amplification. Currently, to amplify two overlapping DNA amplicons, the primer pairs specifically targeting each amplicon are physically separated into different reaction wells, tubes or micro-droplets. For example, BRCA1 and BRCA2 genes contain large exons that require PCR amplification of overlapping DNA amplicons to ensure 100% base coverage. Ion AmpliSeq BRCA1 and BRCA2 Panel, Qiagen GeneRead Human BRCA1 and BRCA2 Panel, and Multiplicom BRCA MASTR Dx separate primer pairs into 3, 4, 5 primers pools, respectively, primarily due to the inability to amplify overlapping amplicons efficiently. However, multiple primer pools significantly complicate the workflow and increase the cost of testing. RainDance Technologies overcomes this issue by separating PCR primers into thousands of micro-droplets, but on a special expensive instrument.

Combining all PCR primers for two overlapping DNA regions in one multiplex reaction produces four products resulting from four different combinations of the two forward primers with the two different reverse primers. FIG. 1 shows a conventional PCR method. The four PCR products (FIG. 1) are two targeted amplicons (Amplicon 1 and Amplicon 2), one long amplicon (Amplicon 4_long) spanning the entire region of the two targeted amplicons and one short amplicon (Amplicon 3_overlap) containing only the overlapped regions between the two targeted amplicons. Using conventional primer design and PCR conditions, during cycling, the longest amplicon (Amplicon 4_long) serves as DNA template for all four amplicons' amplification, and each of the two targeted amplicons (Amplicon 1 & 2) serves as DNA template for amplification of its own amplicon as well as the shortest amplicon (Amplicon 3_overlap). Assuming that all amplifications occur at 100% efficiency, at PCR cycle n, the amount of the four products—Amplicon 1, Amplicon 2, Amplicon 4 long and Amplicon 3 overlap—will be n×2^(n), n×2^(n), 2^(n), and n²×2^(n), respectively. The amount of shortest amplicon (Amplicon 3_overlap) is n times higher than that of each of the two targeted amplicons (Amplicon 1 & 2) which in turn is n times higher than the amount of the longest amplicon (Amplicon 4_long).

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates a traditional multiplex PCR (prior art), for amplification of overlapping target segments.

FIG. 2 illustrates one embodiment of a first round of PCR of the present invention, which is the amplification by target-specific primers. F1 and R1 are the forward and reverse primers of target segment 1, while F2 and R2 are the forward and reverse primers of target segment 2. F2A is a partial sequence of the 5′-end portion of the F2 primer. Tag oligomers of t1, t2 and t3 do not bind to the target. Tags t2 and t3 may share the same sequences and t1 is different. 1, 2, 3 and 4 indicate the amplification products from the combination of four primers. Amplification of Amplicon 3, the short products from F2 and R1, is inhibited by the stem structure that are formed by the two complementary sequences of t1F2A at the two ends of the same strand of Amplicon 3.

FIG. 3 shows the sizes and locations of Amplicons 1-4 on human chromosome 13 for Examples 1 and 2.

FIG. 4 shows the agarose gel electrophoresis results of the amplification products after the first round of PCR (Examples 1 and 2).

FIG. 5 is a schematic illustration of the gene-specific amplicons used in examples 3-6. CDS: protein coding sequence region of a gene; ROI: region of interest. In this example, ROI includes the CDS of BRCA2 gene exon 27 plus 20 bp upstream and 20 bp downstream of exon 27 (CDS ±20 bp).

FIG. 6 shows the agarose gel electrophoresis results of amplification products after the first round (Example 3) and second round of PCR (Example 4).

FIG. 7 is a chart showing the coverage data from NGS results of amplicons (Example 6).

DETAILED DESCRIPTION OF THE INVENTION Definition

An “amplicon” is a piece of DNA or RNA that is the source and/or product of natural or artificial amplification or replication events. In this context, “amplification” refers to the production of one or more copies of a genetic fragment or target sequence, specifically the amplicon. As the product of an amplification reaction, amplicon is used interchangeably with common laboratory terms, such as PCR product.

“Locked nucleic acids” (LNA™) are a class of high-affinity RNA analogues in which the ribose ring is “locked” by a methylene bridge connecting the 2′-O atom and the 4′-C atom resulting in the ideal conformation for Watson-Crick binding. This modification significantly increases the melting temperature of an oligonucleotide and is also very nuclease resistant (www.exiqon.com/lna-technology).

“Peptide nucleic acids” (PNAs) are synthetic homologs of nucleic acids in which the phosphate-sugar polynucleotide backbone is replaced by a flexible pseudo-peptide polymer to which the nucleobases are linked. Because the PNA strand is uncharged, a PNA-DNA duplex will have a higher melting temperature than the corresponding DNA-DNA duplex.

“Universal (PCR) primers” are non-target specific primers that hybridize to universal tags (non-target specific tags such as e.g. t1, t2 and/or t3 in FIG. 2) flanking both ends of any DNA insert sequences. PCR that uses universal primers can amplify any DNA inserts that are flanked by their complementary tag sequences.

The inventors have discovered a scalable multiplex PCR technology, SLIMAMP™ (Stem-Loop Inhibition-Mediated Amplification), which allows for parallel amplification of hundreds of thousands of amplicons in one tube. This novel multiplex PCR method can simultaneously amplify overlapping amplicons without the drawbacks of conventional multiplex PCR, which predominantly amplifies the short overlapping nucleic acid sequences. The present method is a target enrichment method, which enriches copies of target amplicons over copies of the overlapping regions of amplicons.

The present invention is directed to a method for selectively amplifying target nucleic acid fragments having an overlapping region, without a predominant amplification of the short overlapping nucleic acid sequences. The present invention allows all primers in a single primer pool without introducing any additional expensive equipment.

The present invention is illustrated in FIG. 2. FIG. 2 is for illustration purposes and is not meant to limit the invention to the drawings only. The arrangement of tags, forward primers, reverse primers, target nucleic acids, amplicons as described in steps (a)-(d) is shown at the upper part of FIG. 2. The 3′-end of the first target nucleic acid Segment 1 overlaps with the 5′-end of the second target nucleic acid Segment 2.

The present method comprises the steps of: (a) obtaining a first nucleic acid sequence comprising a first tag t2 and a first forward primer F1, (t2F1), complementary to a first target nucleic acid fragment (amplicon 1), (b) obtaining a second nucleic acid sequence comprising a second tag t1 and a first reverse primer R1, (t1R1), complementary to the first target nucleic acid fragment (amplicon 1), (c) obtaining a third nucleic acid sequence comprising the second tag t1 and a second forward primer F2, (t1F2), complementary to a second target nucleic acid fragment (amplicon 2), (d) obtaining a fourth nucleic acid sequence comprising a third tag t3 and a second reverse primer R2, (t3R2), complementary to the second nucleic acid fragment (amplicon 2), wherein the first and the second target nucleic acid fragments have an overlapping region, (e) mixing the first and the second target nucleic acid fragments, the first, the second, the third, and the fourth nucleic acid sequences, and an effective amount of reagents necessary for performing a polymerase chain reaction (PCR); (f) cycling the mixture of (e) through denaturing, annealing and primer extension steps of PCR for at least two times, and (g) cycling the mixture of (f) through denaturing, annealing and primer extension steps of PCR at an annealing temperature higher than that in step (f) to obtain amplification products. The above method steps describe a first round of PCR cycles. The first and the second nucleic acids are designed to amplify Segment 1. The third and the forth nucleic acids are designed to amplify Segment 2.

F1, R1, F2, R2 are gene-specific primers, which are complementary to specific regions of genomic DNA. The length of these primers can be chosen by a person skilled in the art. In general, the gene-specific primers are 6-40, 10-50, 10-40, 10-100, 20-40, or 20-50 nucleotides in length.

Tags t1 and t2 are two different universal tag sequences. Tag t3 can have the same or different sequence as t2. Tag oligomers of t1, t2 and t3 do not bind to the target sequences. Each tag is at the 5′end of each gene-specific primer.

FIG. 2 illustrates one embodiment of the present invention, in which F2̂ is a partial sequence of the 5′-end portion of the F2 primer. In other embodiments of the invention, F2̂ is replaced with 0 nucleotide (not present) or replaced with F2 (full sequence of F2 primer) in FIG. 2. In step (f), the mixture of nucleic acids and reagents go through the PCR cycles of denaturing, annealing and primer extension steps at least two times, such as 2-5 times or 2-10 times, at standard PCR temperatures or conditions known to a person skilled in the art. In the very first PCR cycle, amplicons tagged only at one end are generated. In the second PCR cycle, the one-ended tagged amplicons then serve as templates for the other tagged primers to generate 2-ended tagged amplicons. Step (f) is illustrated at the middle part of FIG. 2, wherein Amplicon 1 (F1+R1), Amplicon 2 (F2+R2), Amplicon 4_long (F1+R2), and Amplicon 3_overlap (F2+R1) are generated after the initial cycles. After at least two cycles of PCR, complete amplicons with the tag sequences at both ends are generated and ready for the next round of PCR cycles with an increased annealing temperature of step (g).

In step (g), the mixture of (f) goes through more cycles of PCR of denaturing, annealing and primer extension; this time at an annealing temperature higher than that in step (f), which is an important feature of the present invention. The annealing temperature is increased to prevent the shortest amplicon (Amplicon 3_overlap) to be amplified predominantly.

If the annealing temperature is not increased (as in a conventional PCR method), all amplification follows a conventional way where Amplicons 1, 2, and 4_long can all serve as templates for the amplification of the short amplicon (Amplicon 3_overlap); this is because Amplicons 1, 2, and 4_long all contain the gene-specific parts of the forward and reverse primers of Amplicon 3_overlap. As a result of the amplification (without increasing annealing temperature), the amount of the shortest amplicon (Amplicon 3_overlap) is n²×2^(n), which is n times higher than that of each of the two targeted amplicons (Amplicons 1 & 2, n×2^(n)), assuming all amplicons have the same amplification efficiencies. In practice, it is observed that amplification efficiency is affected by amplicon length in which shorter amplicons correlate with higher amplification efficiency (Mallona I, et al, BMC Bioinformatics 2011, 12:404), and therefore, the shorter amplicon amplifies even more favorably.

In step (g) of the present invention, the annealing temperature is increased, and thus a successful primer-template hybridization requires not only the gene-specific parts of the primers, but also the tag parts. When the annealing temperature is increased, Amplicons 1, 2, and 4_long can no longer serve as templates for the amplification of the short amplicon (Amplicon 3_overlap), and only the short amplicon itself (Amplicon 3_overlap) can serve as a template for itself. The present invention features (i) an increasing annealing temperature after the initial at least two PCR cycles, and (ii) a proper arrangement of tags (t1, t2, t3) associated with each amplicon, in particular, F2 and R1 primers are tagged with the same tag t1; such features result in production of 2^(n) copies of all amplicons in theory, because each amplicon can only use its own amplicon as a template for its amplification. This is already an improvement over a conventional method, in which Amplicon 3 (short overlap) would yield n times higher than that of target amplicons. In practice, shorter amplicons typically amplify more efficiently than longer amplicons.

In step (g), the annealing temperature is at least 2° C. higher than the annealing temperature in step (f). For example, the annealing temperature is about 2-35° C., 4-35° C., 5-25° C., 6-20° C., 6-15° C. higher than the gene-specific annealing temperature in step (f). In step (g), the PCR cycling is repeated at least 2 times, e.g., 2-50 times, preferably 2-5, 5-10, 10-30, 10-40, or 10-50 times.

Any added bases can increase the primer's melting temperature when the primer sequences match the template 100%. In the present invention, the tag sequences are at least 2 or 3 nucleotides in length, and can be 5-100, 3-40, 10-30, 10-40, 10-50 nucleotides long. Preferably, tags are designed to add at least 5° C. (e.g., 5-10° C. or 5-15° C.) to the melting temperature of the gene-specific untagged primers. The tag sequences provide a higher annealing temperature of the cycling conditions after the initial minimal 2 cycles in PCR. Tag sequences can be modified or unmodified nucleic acids. Some modified bases (e.g. LNA or PNA) have higher annealing temperatures than their corresponding natural bases. When shorter tag sequences are desired for various reasons, those modified bases can be used instead of the natural bases.

In one embodiment of the invention, the Tm (melting temperature) of tag t1 sequences on both ends of amplicon 3 is high enough to form a tight t1-stem, which prevents the hybridization of primer t1F2 to the amplicon 3 template, and inhibits a further exponential amplification of amplicon 3. In order to inhibit the binding of primer t1F2 to amplicon 3 template, the Tm of the t1-stem at the end of amplicon 3 should be the same or higher than the Tm of t1F2 oligo alone. The melting and hybridization of t1-stem at the ends of amplicon 3 follow intramolecular kinetics, and are more favorable than those of the regular intermolecular oligo duplex reactions. Therefore, the same two short complimentary oligo sequences (e.g. 2 to 100 nucleotides) have a much higher Tm when they form a stem connected by a non-complimentary loop in one molecule than Tm of the same two complimentary oligo sequences forming a linear duplex. In addition, the Tm of the stem is not only influenced by the stem sequences but also the loop length (corresponding to the overlapping region). In comparison to a small loop size (e.g. 2-200, 5-200, or 10-100 nucleotides), a large loop size (e.g. greater than 500 nucleotides such as 500-1000 or 500-1500 nucleotides) reduces the stem hybridization rates, possibly due to the decreased probabilities of contact between the ends of the larger loop, and resembles the kinetics of regular intermolecular DNA duplex formations.

In one embodiment, the second nucleic acid sequence optionally further comprises a full sequence (F2), or a partial sequence (F2̂) consecutively from the 5′end of the second forward primer (F2), in between the second tag (t1) and the first reverse primer (R1). In FIG. 2, this optional embodiment of primer t1F2̂R1 is shown. Dependent on the length of F2, in one embodiment, the partial sequence of F2̂ is 1-50, 1-20, 1-10, or 1-5 nucleotides shorter than F2. In one embodiment, the partial sequence of F2̂ is at least 3, at least 4, or at least 5 nucleotides. For example, the partial sequence of F2̂ may be 2-40, 3-40, 4-40, 5-40, 8-40, 8-30, 8-20, 3-35, 4-35, or 5-35 nucleotides. In another embodiment, the partial sequence of F2̂ contains 10-50, 10-90, 20-80, 20-90, 30-70, 40-90, or 50-90% of the F2 sequence. In this embodiment, F2 or F2̂ is inserted directly between the gene-specific sequence of R1 and its tag sequence t1.

In another embodiment, the third nucleic acid sequence optionally further comprises a full sequence (R1), or a partial sequence (R1̂) consecutively from the 5′end of the first reversed primer (R1), in between the first tag (t1) and the second forward primer (F2). Dependent on the length of R1, in one embodiment, the partial sequence of R1^(̂) is 1-50, 1-20, 1-10, or 1-5 nucleotides shorter than R1. In one embodiment, the partial sequence of R1̂ is at least 3, at least 4, or at least 5 nucleotides. For example, the partial sequence of R1̂ may be 2-40, 3-40, 4-40, 5-40, 8-40, 8-30, 8-20, 3-35, 4-35, or 5-35 nucleotides. In another embodiment, the partial sequence of R1̂ contains 10-50, 10-90, 20-80, 20-90, 30-70, 40-90, or 50-90% of the R1 sequence. In this embodiment, R1 or R1̂ is inserted directly between the gene-specific sequence of F2 and its tag sequence t1.

As illustrated at the lower part of FIG. 2, after step (g), amplicon 1 (F1+R1), Amplicon 2 (F2+R2), and Amplicon 4_long (F1+R2) are amplified exponentially by PCR, while the amplification of Amplicon 3_overlap (F2+R1) is inhibited. In FIG. 2, adding t1 to the 5′-end of F2 primer and adding t1F2̂ at the 5′end of R1 primer result in the same strand of Amplicon 3_overlap (F2+R1) being flanked by the complementary sequences of t1 and F2̂ at the two ends of the same strand. Consequently, at the subsequent annealing steps during cycling, the two t1-F2̂ complementary sequences in Amplicon_3 form a strong stem that renders the sequences inaccessible for the hybridization of the forward primer (t1-F2), thus preventing Amplicon 3 from serving as a template for further amplification. The proper length of F2̂ that enables a strong stem structure may vary depending on the Tm of tag t1. FIG. 2 illustrates the stem loop formation with t1 and F2A, which is a preferred embodiment. In another embodiment, as described above, the Tm of tag t1 sequences on both ends of amplicon 3 are high enough to form a tight t1-stem, and the presence of F2̂ is not required.

In another embodiment of the invention, the above amplification products after the first round of PCR as described above are amplified further by a second round of PCR amplification with universal primers that bind to t1 and t2 in the first round. In this embodiment, t3 is required to be the same as t2 during the first round of PCR. The second PCR round comprises the steps of: (h) mixing the amplification products from (g), either treated or untreated, with a first and a second universal PCR primers that bind to t1 and t2 respectively, and an effective amount of reagents necessary for performing a PCR, wherein both universal PCR primers do not comprise sequences complementary to the first and the second target-specific nucleic acid sequences; and (i) cycling the mixture of (h) through denaturing, annealing and primer extension steps of PCR at standard PCR temperatures and conditions known to a person skilled in the art to obtain second amplification products.

In step (i), the PCR cycling is repeated at least 2 times, e.g., 5-50 times, preferably 2-5, 5-10, 10-30, 10-40, or 10-50 times.

During the second round of PCR, the amplifications of the short overlapping Amplicon 3_overlap and the long Amplicon 4_long are further inhibited because their primer binding sites are blocked by the strong stem formation of t1 at both ends of the Amplicon 3_short and t2 at both ends of the Amplicon 4_long. Therefore, the amplifications of the first and the second target nucleic acid amplicons (Amplicons 1 and 2) dominate in the second round of PCR. The final products can be used for different purposes, such as next generation sequencing (NGS).

In the second round of PCR, the product of (g) from the first round of PCR is optionally pre-treated before the step (i), for example, by dilution, single-strand exonuclease digestion, purification, or adaptor ligation.

The following examples further illustrate the present invention. These examples are intended merely to be illustrative of the present invention and are not to be construed as being limiting.

EXAMPLES

Table 1 and FIG. 3 show the sizes and locations of Amplicons 1-4 on human genome hg19. Amplicon 3 is the overlap between Amplicons 1 and 2. Amplicon 4 is the long amplicon covering the sequences of both Amplicons 1 and 2.

TABLE 1 Size Gene- Size With Chr. Specific Tag Amplicon Name Gene hg19 Start hg19 End hg19 (bp) (bp) Amplicon1 BRCA2 chr13 32914076 32914572 497 538 Amplicon2 BRCA2 chr13 32914378 32914737 360 401 Amplicon3_overlap BRCA2 chr13 32914378 32914572 195 236 Amplicon4_long BRCA2 chr13 32914076 32914737 662 703

Table 2 shows oligonucleotide sequences used in Examples 1 and 2, and FIG. 4 SEQ ID NOs: 1-4 are gene specific primers for BRCA2 amplicon 1 and amplicon 2 without tag sequences. SEQ ID NOs: 5-6 are tag sequences from Illumina TSCA tags. SEQ ID NOs: 7-16 are the tagged primers used in the example experiments.

SEQ ID NO: 7 was used in standard multiplex PCR only (FIG. 4, Lane 3). SEQ ID NOs: 8-14 were used in the present invention (Lane 4-8). SEQ ID NOs: 15-16 were used in both the standard multiplex PCR and the present invention (Lane 3-8).

Tm are evaluated by OligoAnalyzer 3.1 (IDTDNA) with 1.5 mM of Mg⁺² concentration.

TABLE 2 Stem_Tm_ formed between t1_F2{circumflex over ( )}_R1 and Oligo Gene Tagged t1_F2, or SEQ ID Direction Specific Full between t1_R1 NO: Sequence* Size (nt) Amplicon Oligo-Tm Oligo Tm and t1_R1{circumflex over ( )}_F2  1: F1 GTGAAAGACATATTTACAGACAGTT FWD 27 Amplicon1 60.6 N/A N/A TC  2: R1 CTTGTGAGCTGGTCTGAATGT REV 21 Amplicon1 61.9 N/A N/A  3: F2 AGGGAAGCTTCATAAGTCAGTC FWD 22 Amplicon2 61.3 N/A N/A  4: R2 TCCTCTAACACTCCCTTAACTTTGT REV 25 Amplicon2 63.1 N/A N/A  5: t1 caacgatcgtcgaaattcgc 20 Tag 61.3 TagOnly N/A N/A  6: t2 tacacgacgctcttccgatct 21 Tag 64.4 TagOnly N/A N/A  7: t1_F1 caacgatcgtcgaaattcgc FWD 47 Amplicon1 60.6 72.6 N/A GTGAAAGACATATTTACAGACAGTT TC  8: t2_F1 tacacgacgctcttccgatct FWD 48 Amplicon1 60.6 73.2 N/A GTGAAAGACATATTTACAGACAGTT TC  9: t2_R1 tacacgacgctcttccgatct REV 42 Amplicon1 61.9 75.2 N/A CTTGTGAGCTGGTCTGAATGT 10: t1_R1 caacgatcgtcgaaattcgc REV 41 Amplicon1 61.9 74.7 63.3 (F2{circumflex over ( )} = 0) CTTGTGAGCTGGTCTGAATGT 11: t1_ caacgatcgtcgaaattcgc AGGG REV 45 Amplicon1 61.9 76.5 75.2 F2{circumflex over ( )}4_R1 CTTGTGAGCTGGTCTGAATGT 12: t1_ caacgatcgtcgaaattcgc AGGGAAGC REV 49 Amplicon1 61.9 77.2 77.2 F2{circumflex over ( )}8_R1 CTTGTGAGCTGGTCTGAATGT 13: t1_ caacgatcgtcgaaattcgc AGGGAAGCTTCA REV 53 Amplicon1 61.9 77.2 77.8 F2{circumflex over ( )}l2_R1 CTTGTGAGCTGGTCTGAATGT 14: t1_ caacgatcgtcgaaattcgc REV 57 Amplicon1 61.9 77.3 77.5 F2{circumflex over ( )}16_R1 AGGGAAGCTTCATAAG CTTGTGAGCTGGTCTGAATGT 15: t1_F2 caacgatcgtcgaaattcgc FWD 42 Amplicon2 61.3 73.4 N/A (R1{circumflex over ( )} = 0) AGGGAAGCTTCATAAGTCAGTC 16: t2_R2 tacacgacgctcttccgatct REV 46 Amplicon2 63.1 74.5 N/A TCCTCTAACACTCCCTTAACTTTGT 26: t1_ caacgatcgtcgaaattcgc CTTGTGAG FWD 50 Amplicon2 61.3 67.7 69.1 R1{circumflex over ( )}8_F2 AGGGAAGCTTCATAAGTCAGTC 27: t1_ caacgatcgtcgaaattcgc CTTGTGAGCTGG FWD 54 Amplicon2 61.3 69.3 72.2 R1{circumflex over ( )}12_F2 AGGGAAGCTTCATAAGTCAGTC *Lower case indicates tag sequences; Underline indicates inserted partial F2 sequences (F2{circumflex over ( )}), or inserted partial R1 sequences (R1{circumflex over ( )}); un-labeled upper case sequences are gene-specific sequences. N/A: Not applicable. FWD: forward. REV: reverse

Table 3 shows primer mix information in Examples 1 and 2.

TABLE 3 F2_R1 Short Amplicon Multiplex Primer Mix Loop Length (nt); Stem Lane Name Amplicon1 Amplicon2 Oligo; Stem length (nt) 1 M: 100 bp Ladder 2 Singleplex Control Not applicable mix^(a) 3 STD_Multiplex_ctrl t1_F1 t2_R1 t1_F2 t2_R2 no stem loop 4 Stem_t1-F2{circumflex over ( )}0 t2_F1 t1_R1 t1_F2 t2_R2 195; t1_only; 20 5 Stem_t1-F2{circumflex over ( )}4 t2_F1 t1_F2{circumflex over ( )}4_R1 t1_F2 t2_R2 195; t1 + 4 nt of F2; 24 6 Stem_t1-F2{circumflex over ( )}8 t2_F1 t1_F2{circumflex over ( )}8_R1 t1_F2 t2_R2 195; t1 + 8 nt of F2; 28 7 Stem_t1-F2{circumflex over ( )}12 t2_F1 t1_F2{circumflex over ( )}12_R1 t1_F2 t2_R2 195; t1 + 12 nt of F2; 32 8 Stem_t1-F2{circumflex over ( )}16 t2_F1 t1_F2{circumflex over ( )}16_R1 t1_F2 t2_R2 195; t1 + 16 nt of F2; 36  9^(b) Stem_t1-R1{circumflex over ( )}8 t1_F1 t1_R1 t1_R1{circumflex over ( )}(=8)_F2 t2_R2 195; t1 + 8 nt of R1; 28 10^(b ) Stem_t1-R1{circumflex over ( )}12 t2_F1 t1_R1 t1_R1{circumflex over ( )}(=12)_F2 t2_R2 195; t1 + 12 nt of R1; 3 ^(a)Contains each singleplex product from separate amplifications mixed at the equal volume ratio ^(b)Data not shown

Example 1 First Round of PCR Amplification (Gene-Specific Primers)

A representative PCR mixture of 25 μL included the following components: 12.5 μL of 2× Multiplex Master Mix (KAPA Biosystems, Cat#KK5802), 2 μL human genomic DNA (Promega, Cat#G3041) diluted to 5 ng/μL in low TE buffer (USB, Cat#75793), 6.5 μL nuclease-free water, and 4 μL of gene-specific primer mix (1.25 μM each, see Multiplex Primer Mix Lanes 3-8 in Table 3).

The singleplex (FIG. 4, Lane 2) PCR, the standard multiplex (FIG. 4, Lane 3) PCR, and the stem-forming multiplex (FIG. 4, Lanes 4-8) PCR were all performed on a thermal cycler as follows:

 1 cycle 95° C.  2 minutes Enzyme activation and initial DNA denaturation  5 cycles 95° C. 30 seconds Denaturation 60° C. 90 seconds Annealing/extension 30 cycles 95° C. 30 seconds Denaturation 72° C. 90 seconds Annealing/extension at an increased temperature  1 cycle 72° C.  5 minutes Final extension  1 cycle  8° C. Hold

Example 2 Agarose Gel Electrophoresis

The products from example 1 were analyzed on an E-base device (Life Technologies). Two μL of the product was diluted to a final volume of 20 μL with nuclease-free water and loaded onto a 2% SizeSelect E-gel. DNA electrophoresis of diluted PCR products (Lanes 2-8) and 1 Kb Plus DNA ladder (Invitrogen, Cat#10488-090, Lane 1) was performed, and at the end of the run, a digital image of the gel was captured by an E-gel Imager (Life Technologies). Results are shown in FIG. 4.

In Lane 2 of FIG. 4, equal amounts of the products of singleplex reactions from Example 1 are mixed and can be seen together on the gel, designated Amplicons 1 and 2. Standard multiplex PCR predominantly produces Amplicon 3 (Lane 3), which is PCR product amplified from the overlap of Amplicons 1 and 2. Amplicon 4, the entire region covered by Amplicons 1 and 2, can faintly be seen in Lane 3. Lanes 4 and 5, which contain stem oligonucleotides of t1 only and t1 plus partial F2 (4 nucleotides from the 5′end), respectively, show similar patterns with three detectable bands: Amplicons 1, 2 and 3. In Lanes 6-8, all Amplicons 1, 2, and 3 can be seen, but amplicon 3 has decreased substantially relative to Lanes 3-5.

Table 4 and FIG. 5 show the amplicon sizes and locations on human genome hg19 targeting BRCA2 Exon 27 (B2X27) used in examples 3-6.

TABLE 4 Amplicon Size Amplicon Size Amplicon Name chr_hg19 Start hg19 End hg19 Gene-Specific (bp) With Tag (bp) B2X27A1 chr13 32972189 32972455 267 308 B2X27A2 chr13 32972396 32972719 324 365 B2X27A3 chr13 32972638 32972985 348 389 Overlap of B2X27A1 + A2 chr13 32972396 32972455 60 101 Overlap of B2X27A2 + A3 chr13 32972638 32972719 82 123

Table 5 shows oligonucleotide sequences used in Examples 3, 4, 5, and 6 for BRCA2 (B2) Exon 27 (X27) amplification.

TABLE 5 Oligo Direction SEQ ID NO: Sequence* Size (nt) Amplicon 17: caacgatcgtcgaaattcgc ACTGTGTGTAATATTTGCGTGCTT FWD 44 B2X27A1 t1_B2X27A1F 18: tacacgacgctcttccgatct TTCTCTTTTTGCAGTTCTTTTGGT REV 45 B2X27A1 t2_B2X27A1R 19: caacgatcgtcgaaattcgc CTTCAAAGTCTTGTAAAGGGGAGA FWD 44 B2X27A2 t1_B2X27A2F 20: tacacgacgctcttccgatct TTGAACCAGACAAAAGAGCTTG REV 43 B2X27A2 t2_B2X27A2R 21: caacgatcgtcgaaattcgc GAAATTTCTCTTTTGGAAAGTAATTCA FWD 47 B2X27A3 t1_B2X27A3F 22: REV 48 B2X27A3 t2_B2X27A3R tacacgacgctcttccgatct TGTGGTTTGAAATTATATTCCAGTCTT 23: t2_ tacacgacgctcttccgatct CTTCAAAGTC REV 55 B2X27A1 B2X27A1.SLIM. TTCTCTTTTTGCAGTTCTTTTGGT R 24: t2_ tacacgacgctcttccgatct CTTCAAAGTCTTGTAAAGGGGAGA FWD 45 B2X27A2 B2X27A2.SLIM. F 25: t1_ caacgatcgtcgaaattcgc GAAATTTCTCT REV 53 B2X27A2 B2X27A2.SLIM. TTGAACCAGACAAAAGAGCTTG R *Lower case indicates tag sequences; Underline indicates inserted partial forward primer sequences from the next amplicon; un-labeled, upper case sequences are gene-specific sequences.

Example 3 First Round of Gene-specific PCR Amplification for BRCA2 exon 27 (B2X27)

A representative PCR mixture of 25 μL included the following components: 12.5 μL of 2× Multiplex Master Mix (KAPA Biosystems, Cat#KK5802), 6 μL of DNA (Coriell, Cat#NA19240 or NA14622) diluted to 5 ng/μL in low TE buffer (IDT, Cat#11-05-01-09), 2.5 μL nuclease-free water, and 4 μL of gene-specific primer mix (1.25 μM each).

The conventional primer mix contained the following six oligos from Table 5: SEQ ID NOs: 17 and 18 (B2X27 amplicon 1), 19 and 20 (B2X27 amplicon 2), and 21 and 22 (B2X27 amplicon 3). The SLIMAMP™ primer mix (the present invention) contains the following six oligos: 17 and 23 (B2X27 amplicon 1), 24 and 25 (B2X27 amplicon 2), and 21 and 22 (B2X27 amplicon 3).

The standard multiplex and stem-forming multiplex PCR is performed on a thermal cycler as follows:

 1 cycle 95° C.  2 minutes Enzyme activation and initial DNA denaturation  5 cycles 95° C. 15 seconds Denaturation 60° C.  6 minutes Annealing/extension 25 cycles 95° C. 30 seconds Denaturation 72° C.  3 minutes Annealing/extension at increased temperature  1 cycle  8° C. Hold

Example 4 Second Round of PCR Amplification (Universal Primers) and Purification

The gene-specific products in Example 3 were diluted 1000-fold. A representative PCR mixture of 25 μL included the following components: 2.5 μL 10× reaction buffer, 0.5 μL dNTPs, 0.25 μL enzyme (Roche, Cat#12140314001), 2 μL of the diluted product from Example 3, 2 μL of Illumina Index Forward primer, 2 μL Illumina Index Reverse primer (25 μM primer stock from TruSeq Custom Amplicon Index Kit, Cat#FC-130-1003), and 15.75 μL nuclease-free water.

PCR amplification was performed as follows:

 1 cycle 95° C.  4 minutes Initial DNA denaturation 95° C. 30 seconds Denaturation 20 cycles 66° C. 30 seconds Annealing 72° C. 60 seconds Extension  1 cycle 72° C.  5 minutes Final extension  1 cycle  8° C. Hold

The product was purified by adding 18 μL of Agencourt AMPure XP beads (Beckman Coulter, Cat#A63881), separating the beads from the supernatant, and discarding the supernatant. Two washes of 70% ethanol were used to wash the beads, and the product was eluted from the beads using 32 μL nuclease-free water. The concentration of the product was then quantified using 2 μL of the product diluted in 198 μL of Qubit High Sensitivity buffer (Invitrogen, Cat#Q32854).

Example 5 Agarose Gel Electrophoresis

The products from example 3 and example 4 were analyzed on an E-base device (Life Technologies). Two μL of the product was diluted to a final volume of 20 μL with nuclease-free water and loaded onto a 2% SizeSelect E-gel. DNA electrophoresis of diluted PCR products and 50 bp DNA ladder (Invitrogen, Cat# 10488-043) was performed. At the end of the run, a digital image of the gel was captured by an E-gel Image (Life Technologies). Results are shown in FIG. 6.

The products from example 3 (First round of PCR products) are shown in Lane 1-4 of FIG. 6. Lanes 1 and 2 demonstrate the products of conventional multiplex PCR. The small amplicons, which are the overlaps of amplicons 1 with 2 and 2 with 3, were produced along with amplicons 1-3. On the other hand, SLIMAMP™ reactions, loaded in Lanes 3 and 4, produce mainly amplicons 1-3, with very little of the overlapping amplicons.

The products from Example 4 (second round of PCR products) are shown in Lanes 5-8 of FIG. 6, which are products after universal PCR and cleaned up by beads. In Example 4 (Lanes 5-8), amplicons were tagged and amplified further, demonstrated by the increase in nucleotide size when compared with Lanes 1-4. The samples that originally underwent conventional multiplex PCR produced primarily the undesired short overlapping amplicons (Lanes 5-6), while the SLIMAMP™ samples (the present invention) contain only the targeted amplicons 1-3 (Lanes 7-8).

Example 6 NGS Library Normalization and Sequencing

Each product from Example 4 was normalized to 4 mM using 10 mM Tris-HCl w/0.1% Tween 20 (Teknova, Cat#T7724). All normalized products from Example 4 are mixed in equal volume (3 μL each) to create a library mix. 5 μL of the library mix is added to 5 μL of 0.2 N NaOH to denature the library. A 20 pM library is prepared using HT1 buffer, loaded, and sequenced with a 250 bp paired-end read length (Illumina, Cat#MS-102-2003). The resulting Fastq sequencing reads for each sample was then aligned to the hg19 reference genome by BWA-MEM. The paired-end reads were then merged and the coverages for each amplicon regions were analyzed. The coverage information is shown in FIG. 7.

FIG. 7 depicts the percentage of NGS reads attributed to each amplicon in each sample. In the conventional multiplex PCR, the overlapping amplicons were amplified more efficiently than the target amplicons and count for 78% of the total NGS reads, due to their much smaller sizes (60 and 82 bp). In the SLIMAMP™ samples, all the undesired overlapping amplicons were inhibited with close to 0% detected in NGS. With respect to the targeted amplicons in both the conventional and SLIMAMP™ samples, as expected, amplicon 1 (267 bp) had higher percentage than both amplicons 2 (324 bp) and 3 (348 bp) because a smaller size (amplicon 1) typically amplifies more efficiently.

It is to be understood that the foregoing describes preferred embodiments of the present invention and that modifications may be made therein without departing from the scope of the present invention as set forth in the claims. 

What is claimed is:
 1. A method for selectively amplifying target nucleic acid fragments having an overlapping region, comprising the steps of: (a) obtaining a first nucleic acid sequence comprising a first tag (t2) and a first forward primer (F1) complementary to a first target nucleic acid fragment, (b) obtaining a second nucleic acid sequence comprising a second tag (t1) and a first reverse primer (R1) complementary to the first target nucleic acid fragment, (c) obtaining a third nucleic acid sequence comprising the second tag (t1) and a second forward primer (F2) complementary to a second target nucleic acid fragment, (d) obtaining a fourth nucleic acid sequence comprising a third tag (t3), and a second reverse primer (R2) complementary to the second nucleic acid fragment, wherein the first and the second target nucleic acid fragments have an overlapping region, (e) mixing the first and the second target nucleic acid fragments, the first, the second, the third, and the fourth nucleic acid sequences, and an effective amount of reagents necessary for performing a polymerase chain reaction (PCR); (f) cycling the mixture of (e) through denaturing, annealing and primer extension steps of PCR for at least two times, and (g) cycling the mixture of (f) through denaturing, annealing and primer extension steps of PCR at an annealing temperature higher than that in step (f) to obtain amplification products.
 2. The method according to claim 1, wherein the third nucleic acid sequence further comprises a full sequence of the first reverse primer (R1), in between the first tag (t1) and the second forward primer (F2).
 3. The method according to claim 1, wherein the third nucleic acid sequence further comprises a 5′-end partial sequence of the first reverse primer (R1A), in between the first tag (t1) and the second forward primer (F2).
 4. The method according to claim 3, wherein the 5′-end partial sequence of the first reverse primer contains 10-90% of the R1 sequence.
 5. The method according to claim 1, wherein the annealing temperature in step (g) is 2-35° C. higher than the annealing temperature in step (f).
 6. The method according to claim 1, wherein the PCR cycling in step (f) is repeated 2-10 times.
 7. The method according to claim 1, wherein the third tag is the same as the second tag.
 8. The method according to claim 6, further comprising the steps of: (h) mixing the amplification products from step (g), either treated or untreated, with a first and a second universal PCR primers that bind to t1 and t2 respectively, but do not bind to the first and the second target nucleic acid fragments, and an effective amount of reagents necessary for performing a PCR, and (i) cycling the mixture of step (h) through denaturing, annealing and primer extension steps of PCR to obtain second amplification products.
 9. The method of claim 8, wherein the products from step (g) is pre-treated by dilution, single-strand exonuclease digestion, purification, or adaptor ligation. 